Sintered NdFeB magnets are expected to be in greater demand in the future as a component of the motor of a hybrid car or other devices. Accordingly, a further increase in its coercivity has been demanded. One well-known method for increasing the coercivity HcJ of the sintered NdFeB magnet is to substitute dysprosium (Dy) or terbium (Tb) for a portion of neodymium (Nd). However, Dy and Tb are scarce resources and unevenly distributed. Furthermore, the substitution by these elements decreases the residual magnetic flux density Br and the maximum energy product (BH)max of the sintered NdFeB magnet.
It has recently been found that the HcJ can be increased with almost no decrease in the Br of the magnet by applying Dy or Tb to the surface of the sintered NdFeB magnet by sputtering, and then heating it at a temperature of 700° to 1000° C. (Non-Patent Documents 1 to 3). The Dy or Tb applied to the magnet's surface move through the grain boundary of the sintered compact into the compact's body and diffuse from the grain boundary into each particle of the main phase, R2Fe14B, where R is a rare-earth element (This phenomenon is called grain boundary diffusion.) In this process, since the R-rich phase is liquefied by the heat treatment, the diffusion rate of Dy or Tb within the grain boundary is much faster than their diffusion rate from the grain boundary into the main-phase particle. This difference in the diffusion rate can be utilized to adjust the temperature and time of the heat treatment so as to create, over the entire sintered compact, a state where Dy or Tb is present with high concentration only within a region (surface region) in the vicinity of the grain boundary of the main-phase particle of the sintered compact. The coercivity HcJ of the sintered NdFeB magnet depends on the state of the surface region of the main-phase particle; a sintered NdFeB magnet whose crystal grain has a high concentration of Dy or Tb in the surface region will have a high coercivity. Although the increase in the concentration of Dy or Tb lowers the Br of the magnet, the decrease in the Br of the entire main-phase particle is negligible since this decrease occurs only within the surface region of each main-phase particle. Thus, the resultant product will be a high-performance magnet having a high HcJ value and yet maintaining the Br comparable to that of a sintered NdFeB magnet that has not undergone the substitution by Dy or Tb. This technique is called a grain boundary diffusion method.
Methods for industrially producing a sintered NdFeB magnet by the grain boundary diffusion method have already been made public (Non-Patent Documents 4 and 5): One method includes forming a fine powdered layer of a fluoride or oxide of Dy or Tb on the surface of a sintered NdFeB magnet and heating it; and another method includes burying a sintered NdFeB magnet in a mixed powder composed of the powder of a fluoride or oxide of Dy or Tb and the powder of calcium hydride and heating it.
Substituting Ni or Co for a portion of Fe in a sintered NdFeB magnet improves the corrosion resistance of the magnet; increasing the total substitution percentage of Ni and Co to a level higher than 20 to 30% prevents rusting in the anti-corrosion test (at 70° C., at a humidity of 95%, and for 48 hours) (Non-Patent Document 6). However, using a large amount of Ni and Co increases the price of the magnet, and so it has been difficult to industrially use sintered NdFeB magnets produced by this method.
Relevant techniques were also proposed before the grain boundary diffusion method was publicly known, such as the technique of diffusing at least one of the elements Tb, Dy, Al and Ga in the vicinity of the surface of the sintered NdFeB magnet to suppress the high-temperature irreversible demagnetization (Patent Document 1), or the technique of covering the surface of the sintered NdFeB magnet with at least one of the elements Nd, Pr, Dy, Ho and Tb to prevent the deterioration of the magnetic characteristics due to working degradation (Patent Document 2).    Patent Document 1: Japanese Unexamined Patent Application Publication No. H01-117303    Patent Document 2: Japanese Unexamined Patent Application Publication No. S62-074048    Non-Patent Document 1: K. T. Park et al., “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Sintered NdFeB Magnets”, Proceedings of the Sixteenth International Workshop on Rare-Earth Magnets and their Applications (2000), pp. 257-264    Non-Patent Document 2: Naoyuki Ishigaki et al., “Neojimu Kei Bishou Shouketsu Jishaku No Hyoumen Kaishitsu To Tokusei Koujou (Surface Modification and Characteristics Improvement of Micro-sized Neodymium Sintered Magnet)”, NEOMAX GIHOU (NEOMAX Technical Report), published by Kabushiki Kaisha NEOMAX, vol. 15 (2005), pp. 15-19    Non-Patent Document 3: Ken-ichi Machida et al., “Nd—Fe—B Kei Shouketsu Jishaku No Ryuukai Kaishitsu To Jiki Tokusei (Grain Boundary Modification and Magnetic Characteristics of Sintered NdFeB Magnet)”, Funtai Funmatsu Yakin Kyoukai Heisei 16 Nen Shunki Taikai Kouaen Gaiyoushuu (Speech Summaries of 2004 Spring Meeting of Japan Society of Powder and Powder Metallurgy), published by the Japan Society of Powder and Powder Metallurgy, 1-47A    Non-Patent Document 4: Kouichi Hirota et al., “Ryuukai Kakusan Hou Ni Yoru Nd—Fe—B Kei Shouketsu Jishaku No Kou-hojiryoku-ka (Increase in Coercivity of Sintered NdFeB Magnet by Grain Boundary Diffusion Method)”, Funtai Funmatsu Yakin Kyoukai Heisei 17 Nen Shunki Taikai Kouen Gaiyoushuu (Abstracts of Autumn Meeting of Japan Society of Powder and Powder Metallurgy), published by the Japan Society of Powder and Powder Metallurgy, p. 143    Non-Patent Document 5: Ken-ichi Machida et al., “Ryuukai Kaishitsu Gata Nd—Fe—B Kei Shouketsu Jishaku No Jiki Tokusei (Magnetic Characteristics of Sintered NdFeB Magnet with Modified Grain Boundary)”, Funtai Funmatsu Yakin Kyoukai Heisei 17 Nen Shunki Taikai Kouen Gaiyoushuu (Abstracts of Autumn Meeting of Japan Society of Powder and Powder Metallurgy), published by the Japan Society of Powder and Powder Metallurgy, p. 144
Non-Patent Document 6: Yasutala Fukuda et al., “Magnetic Properties and Corrosion Characteristics of Nd—(Fe,Co,Ni)—B Pseudo-Ternary Systems”, KaWASAKI STEEL GIHO (Kawasaki Steel Technical Report), published by Kawasaki Steel Corporation, vol. 21(1989), No. 4, pp. 312-315